Low density lipoproteins (LDLs) play a major role in cholesterol transport in the blood and are risk factors for arteriosclerosis. It is known that small, dense lipoproteins (small, dense LDLs (small-particle, low-density lipoproteins)), which are particularly small in particle size among LDLs and higher in specific gravity compared with standard LDLs, have arteriosclerosis-inducing ability at a level several-fold higher than that of normal LDLs. Increase of small, dense LDLs is one of the major risk factors for arteriosclerosis. It is clinically very important to perform a fractional measurement for such small, dense LDLs.
Examples of conventional methods for measurement of small, dense LDLs include an ultracentrifugation method, an electrophoresis method, and a method using high performance liquid chromatography. These methods are not convenient since they require expensive facilities and much time for measurement.
An example of a method for measuring small, dense LDLs using an autoanalyzer is a method (see JP Patent Publication (Kokai) No. 2003-28882 A) that involves mixing and suspending or dissolving small, dense LDLs with the use of differences in ionic strength and then measuring the small, dense LDLs with the use of differences in absorbance. However, differences in absorbance are measured based on turbidity according to such method, and thus specificity and accuracy are insufficient.
Furthermore, a method (see WO2004/053500) that involves measuring cholesterol or triglycerides in small, dense LDLs through the use of a combination of a separation agent comprising polyanions and divalent cations and a reagent adaptable for an autoanalyzer is known. This method is capable of measuring lipid components in small, dense LDLs more conveniently than an ultracentrifugation method or an electrophoresis method. Furthermore, the method is excellent in specificity and accuracy. However, the method requires pretreatment of specimens and a procedure for separating small, dense LDLs from LDLs other than such LDLs.